1. FIELD OF THE INVENTION
This invention relates to a scanning tunneling microscope.
In the operation of such a microscope, a stroke of about 5 mm is required for the Z-axis coarse drive in order to replace samples easily. On the other hand, a precision of 50 nm/step at most is required in order to convey the microscope needle to the tunneling region at about 2 mm from the sample surface. In the final step when the needle approaches the sample, the tunneling current is detected, the Z-axis coarse drive motor is stopped, and at the same time the precision drive element for the Z direction raises the needle to prevent the sample and needle from colliding. It follows that the operational speed of the precision drive element must be faster than the conveying speed of the Z-axis coarse drive motor. At present, in order to prevent the needle and sample from colliding, the Z-axis coarse drive speed has an upper limit which is set by conveying with a Z-axis coarse drive pulse motor at a repetition rate of less than 200 to 300 Hz. The following three strategies have conventionally been adopted as countermeasures to this:
2. Prior Art 1
While monitoring the space between the needle and sample with a stereomicroscope, the Z drive stage is conveyed at high speed until the needle comes in to close proximity with the sample and thereafter it is switched manually to a low speed.
3. Prior Art 2
On the first occasion, during the initial Z-axis coarse drive, all the conveyance is carried out at low speed and the number of pulses until arriving at the tunneling region are recorded. On the following occasions, the conveyance is carried out at high speed until arriving close to the sample, switching to low speed thereafter.
4. Prior Art 3
The sample is arranged in such a way that the top surface of the sample is held down in such a way that the distance between the needle and the sample is fixed even if the thickness of the sample varies (referred to as regulation of the top surface of the sample hereinbelow). Thus, the switching point between high speed and low speed in the Z-axis coarse drive can be fixed and does not depend upon the thickness of sample.
Prior art 1 makes it necessary to continually monitor the movement of the needle or sample. Further, with prior art 2, the Z-axis coarse drive takes time on the first occasion and, if the thickness of the sample frequently changes, movement has to be carried out at low speed to the same extent, and the throughput is 15 reduced. Moreover, while prior art 3 is acceptable for flat samples, there are problems in that it is difficult to apply when the sample is small and cannot be held down by the top surface of the holder or when there are large recesses and protrusions in the sample.
The present invention overcomes these disadvantages and aims to provide a composite tunneling microscope arranged in such a way that it moves the needle or the sample into the tunneling region by the distance from the STM needle to the sample surface on the basis of the focal length measured by an optical microscope or electron microscope.